In comparison with incandescent and halogen lamps, LEDs have higher efficiency and longer lives. With the recent increase in the intensity of white LEDs, active studies have been made for applying the white LEDs to illumination purposes. Among various point light sources, LEDs are expected, owing to their property, to replace halogen lamps which are currently widely used for spot lighting at shops, museums, and showrooms.
At present, a typical white LED is composed of an LED bare chip that emits blue light and a phosphor that emits yellow light when excited by the blue light. This combination of the LED chip and the phosphor produces white light as a result of color mixture. Generally, such a white LED composed of a bare chip and a phosphor is manufactured by mounting the bare chip on a lead frame or printed wiring board, and dropping a resin mixed with a phosphor material from above. As a result, the phosphor is formed to surround the bare chip (See, for example, JP patent No. 2998696).
Unfortunately, however, according to JP patent No. 2998696, the resulting white LEDs tend to be inconsistent in the color of white light for the following reason. Since the phosphor film of each LED is formed by dropping a resin mixed with a phosphor material onto the LED bare chip, the shape (outer shape) of each phosphor film is not consistent. As a result, the thickness of a phosphor surrounding each LED chip varies. The color temperature of white light emitted by a white LED is determined by the proportion of the amounts of blue and yellow light. When the phosphor film is thicker, less blue light is emitted, so that the proportion of yellow light increases and the resulting white light will be of a lower color temperature. Reversely, when the phosphor film is thinner, the resulting white light will be of a higher color temperature. Accordingly, the white LEDs according to JP patent No. 2998696 suffer from inconsistency in color temperatures.
JP patent application publication No. 2002-185048 discloses a technique of forming a phosphor film into a more consistent shape. According to the disclosure of JP patent application publication No. 2002-185048, after an LED bare chip is mounted on a printed wiring board, a phosphor film is formed by stencil printing. As a result, the phosphor film is formed into a uniform outer shape.
Yet, even if each phosphor film has a uniform outer shape, an LED bare chip may unintentionally rotate during a mounting process, which causes inconsistency in color of emission light. More specifically, since a phosphor film is shaped into a square conforming to a square LED chip, if the LED chip is rotated, the sides of the LED chip are no longer parallel to the sides of the phosphor film at all. As a result, the phosphor film of one LED chip is not uniform in thicknesses measured from different points on the lateral surfaces. Since the angle of such unintentional rotation differs from chip to chip, the color temperatures of white LEDs are not uniform for the same reason as described above.
In view of the above problem, JP patent application 2004-172586 discloses a white LED having a cylindrically shaped phosphor film covering a square LED bare chip. With this structure, even if the LED chip rotates, the resulting white LEDs will have a more uniform color temperature. In addition, the phosphor film will produce a beam with a circular spot profile, which is an advantageous property for use as the illumination light sources mentioned above. This property is also advantageous in optical design when used in combination with a reflector.
As the application of white LEDs to illumination purpose advances, LED chips are upsized in order for a single LED bare chip to emit more light. Conventionally, LED chips are typically 0.3 mm per side square. Now there are LED chips of 2 mm per side square.
With the technique disclosed in JP patent application 2004-172586, the difference in colors of light emitted by white LED chips is suppressed. Yet, when applying the technique to a larger-sized LED chip, the color inconsistency within a single white LED chip exceeds a maximum permissible level.
The present invention aims to provide a semiconductor light emitting device without much color inconsistency, even with a larger-sized light emitting element (LED chip). The present invention also aims to provide a lighting module, an illumination apparatus, a surface mount LED, and a bullet LED each having the semiconductor light emitting device.